In recent years, liquid crystal display devices have been widely used because of their advantageous features, such as thinness, lightness, low driving voltage, and low power consumption. Particularly, active-matrix liquid crystal display devices having an active element for each pixel, such as TFT-LCDs (Thin Film Transistor Liquid Crystal Displays) have been becoming comparable with CRTs in terms of display quality.
However, the use of LCDs has been limited due to a narrow viewing angle. In order to eliminate this problem, various techniques have been suggested. Among those techniques, there are many techniques in which electrodes are patterned so as to control the inclinations of liquid molecules in various directions by changing the field distribution in cells. However, the electrode patterning techniques cause problems described later herein. The present invention can be applied to all of the electrode patterning techniques, and easily solve those problems.
First, LCDs that are generally used for display devices will be described. At present, the most commonly used LCDS are TN (Twisted Nematic) LCDs of normally white mode. FIG. 1A shows the panel structure of such a TN-LCD. In FIG. 1A, TN liquid crystal 12 is sandwiched by glass substrates having alignment layers 10 and 11 having orientated directions deviated by 90 degrees from each other. Accordingly, the liquid crystal in contact with the alignment layers 10 and 11 are aligned in the orientated directions of the alignment layers, and the other liquid crystal molecules are orientated along the aligned liquid crystal molecules. As a result, the molecules are twisted through 90 degrees. Further, the liquid crystal and the alignment layers 10 and 11 are sandwiched by two polarizing plates 13 and 14 that are situated in parallel with the orientated directions of the upper and lower portions of the liquid crystal, respectively. When light impinges on the panel having the above structure, the light passing through the polarizing plate 13 turns into linearly polarized light and enters the liquid crystal 12. Along with the liquid crystal 12 twisted through 90 degrees, the light is also twisted through 90 degrees while passing through the lower polarizing plate 14. Here, the display is in a bright state.
As shown in FIG. 1B, a voltage is applied between the alignment layers 10 and 11, thereby straightening the liquid crystal molecules and eliminating the twist. However, on the surfaces of the alignment layers 10 and 11, the liquid crystal molecules remain along the alignment layers 10 and 11 due to the strong orientation force. In this situation, the liquid crystal 12 is almost homeotropic with the linearly polarized light, and no rotation of the polarizing direction occurs. Here, the display is in a dark state. When the voltage is zero, the display returns to a bright state due to the orientation force y on the alignment layers.
Next, an IPS (In-Plane Switching) method that realizes a wider viewing angle will be described. For instance, Japanese Patent No.53-48542 and Japanese Patent No. 1-120528 disclose methods in which an electric field in parallel with the substrates is generated in liquid crystal layers. As shown in FIG. 2A, according to this method, a pair of striped electrodes 21 and 22 are formed on a substrate 20 on one side, and the liquid crystal molecules located between the slit-like electrodes 21 and 22 are driven by a lateral electric field. The liquid crystal 23 is made of a material having positive dielectric anisotropy. When no electric field is applied, the liquid crystal molecules are homogeneously aligned in parallel with the longitudinal direction of the striped electrodes 21 and 22, as shown in the plan view of FIG. 2B (i.e., the liquid crystal molecules are homogeneously orientated at an angle of approximately 15 degrees, so that the direction of the liquid crystal molecules becomes uniform when a voltage is applied).
When a voltage is applied between the striped electrodes 21 and 22, the directors of the liquid crystal molecules having dielectric anisotropy are changed, as shown in FIGS. 3A and 3B. In such a liquid crystal display device, polarizing plates 25 and 26 are arranged on and under substrates 20 and 24, with the polarizing axes or the absorbing axes crossing perpendicularly to each other. One of the polarizing axes is located in parallel director direction, so that a black display can be realized when no voltage is applied and a white display can be realized when a voltage is applied.
There have been dramatic improvements in the TN-type TFT-LCD production techniques, and, in recent years, the TN-type TFT-LCD production techniques excel CRTs in contrast ratio and color reproducibility. However, the LCDs have a narrow viewing angle. Particularly, a TN-type has only a very narrow viewing angle in the vertical direction. Viewed from some other direction, the brightness of the black state increases, making the image whitish. Viewed from the other direction, the display becomes dark, and gray-scale inversion occurs.
When a voltage is applied to TN liquid crystal cells, the liquid crystal molecules are inclined in some degree. At this point, the birefringence of the liquid crystal layer makes the cells have a gray scale transmittance. However, this is the case only when the liquid crystal panel is seen from the front, and the liquid crystal panel looks different when it is seen from an oblique angle. In FIG. 4, the appearance seen from the left is different from the appearance seen from the right. For instance, the liquid crystal has little birefringence effect on the light directed from the lower left to the upper right in FIG. 4. Accordingly, when seen from the right, the panel looks black, not gray. On the other hand, with the light passing from the lower right to the upper left, the birefringence effect becomes larger. As a result, the display looks even whitish and closer to white.
To solve this problem, it is effective to employ the technique of multidomain. According to this technique, a plurality of inclined directions of the liquid crystal molecules exist in one pixel. Because of this, the left half of the pixel in FIG. 5 exhibits a large birefringence for the light passing from the lower left to the upper right (white display), and the right half of the pixel exhibits little birefringence (white display). In such a condition, the display appears to be a gray scale image to a human eye, as long as the division size is small enough. When seen from the left, the display appears to be gray for the same reason. From seen from the front, the display of course appears to be gray because the inclined angles of the liquid crystal molecules are uniform on both left and right sides. Thus, uniform gray scale image can be obtained throughout a wider viewing angle range.
To achieve a multidomain structure, a mask rubbing process shown in FIG. 6 can be used. When an alignment layer is rubbed with a rubbing roller made of nylon or polyester, the liquid crystal molecules have tendency to be orientated in the rubbing direction. Taking advantage of this tendency, alignment layers 32 and 33 of substrates 30 and 31 are subjected to rubbing in the right direction by rubbing rollers 34 and 35, as shown in FIG. 6A. Next, a half of each pixel is subjected to masking with resists 36 and 37, as shown in FIG. 6B. The alignment layers 32 and 33 are then subjected to rubbing in the left direction, as shown in FIG. 6C. The resists 36 and 37 are then removed, and the substrates 30 and 31 are attached to each other, with liquid crystals inside, thereby completing a liquid crystal cell having the left and right orientation directions.
With the mask rubbing method, however, there exist many problems. These problems include the low productivity due to the complicated process, the limitation on the number of divisions (at least four divisions are necessary to satisfy all the conditions with respect to contrast, color, and gray scale, but the maximum number of divisions is 2 because of the complicated process), and poor controllability in the rubbing process due to the masking process. For these reasons, it has been very difficult to mass-produce the multidomain panels by the mask rubbing process.
Other techniques to solve the above problems and to achieve a wider viewing angle include a method in which an electric field distortion is caused in a cell so as to control the alignments. However, there are other problems in this case, such as the difficulty in patterning electrodes, poor yield, and higher costs due to a larger number of processes. Furthermore, if a minutely striped electrode is formed from an ITO (indium tin oxide) layer, a voltage drop is caused at the end portion of the electrode, resulting in display unevenness.
In the IPS method, the liquid crystals are switched in the horizontal direction. As mentioned before, when the liquid crystal molecules are aligned with an inclined angle to the substrates, the birefringence varies with the viewing angle direction. The switching is carried out in the horizontal direction so as to steady the birefringence and obtain excellent viewing angle characteristics. However, this method also causes several problems. First of all, the response is very slow, because the switching is carried out with an electrode gap of about 10 μm in the IPS method, compared with the switching with an electrode gap of about 5 μm in the conventional TN method. The response time can be shortened by narrowing the electrode gap, but each two adjacent electrodes needs to have a different electrical potential to apply an electrical field. Otherwise, short-circuiting will occur between the adjacent electrodes, resulting in a display with defects. To avoid such a problem, each two adjacent electrodes are formed on two different layers, but this simply increases the number of manufacturing processes.
Also, since it is difficult to form minutely striped electrode with ITO, striped metal electrodes are employed instead. However, this causes a loss in the aperture ratio. If the pitch of the striped electrode is narrowed to increase the response speed, the proportion of the electrode to the total area becomes large, resulting in lower transmittance. (In reality, the transmittance of the IPS method is only two thirds of the transmittance of the TN method. If the electrode pitch is halved and the density of the striped electrode is doubled, the transmittance becomes only one third of the transmittance of the TN method.) Because of the above reasons, the similar display quality (in terms of brightness) cannot be obtained, unless the brightness of the backlight is tripled, for instance.
In reality, when quickly moving dynamic images are displayed, blurring occurs in the images. Further, to increase the response speed, a panel is subjected to rubbing not in the direction of the electrode but in the direction deviated by for example about 15 degrees from the direction of the electrode. If the rubbing is performed in the direction of the electrode, the rotational direction of the liquid crystal molecules in the middle of the electrodes is not stabilized as one direction, resulting in a longer response time. Therefore, by inclining the rubbing direction by 15 degrees, even after this process, the response time is twice that of the TN type. Through this process, the viewing angle characteristics are not perfectly symmetric, and some gray-level inversion occurs around the rubbing direction.
To explain this situation, the coordinate system of a polar angle θ shown in FIG. 7A and the azimuth angle φ shown in FIG. 7B are determined for the substrates 20 and 24, the electrodes 21 and 22, and the liquid crystal molecules 23 shown in FIGS. 7A and 7B. FIG. 8A shows the viewing angle characteristics of the panel, in which the gray scales are divided into eight levels from the white state to the black state, and the brightness variations are analyzed by varying the polar angle and the azimuth angle. The shadowed portions in the figure represent gray-level inversion, which are caused in the two azimuths (the 45-degree ranges of 60 to 105 degrees and 240 to 285 degrees in the azimuth angle φ). FIG. 8B shows the transmittance variations of the 8 gray-level displays with respect to the polar angle θ at an azimuth angle of 75 degrees, which causes inversion. The gray-level inversion is caused due to a drop in the white brightness.
In the IPS method, the gray-level inversion is caused due to a drop in the white brightness in the two azimuths, and the viewing angle characteristics deteriorate. By switching in the lateral direction, viewing angle characteristics equivalent to a multidomain panel, but the transmittance, the response speed, the productivity and the market price are all sacrificed. Particularly, a low response speed is not suitable for displaying dynamic picture images.